The present invention relates to a system for controlling an air-to-fuel ratio in an internal combustion engine. More particularly, the present invention relates to a novel three-way catalyst model in a control feedback loop for an engine air-to-fuel ratio (A/F) control system.
Government regulations concerning the release into the atmosphere of various exhaust emission constituents from automotive vehicles are becoming increasingly more stringent. As the regulations relating to emissions of oxides of nitrogen, carbon monoxide, and unburned hydrocarbons become more stringent, it is necessary to control the engine air-to-fuel ratio (A/F) to avoid unnecessary instabilities and minimize undesirable tailpipe exhaust emissions.
A fore-aft oxygen sensor (FAOS) fuel control system 100 is shown in FIG. 1. The control system 100 uses an A/F bias 102 for trimming the closed-loop operating point of a pre-catalyst A/F feedback controller 104. The A/F bias 102 is generated by a proportional-integral feedback signal from a post-catalyst EGO sensor feedback controller 106 fed by the output of a post-catalyst Exhaust Gas Oxygen (EGO) sensor 108.
A substantial time delay is associated with the post-catalyst feedback loop. Therefore, this control system 100 is subject to some A/F errors. A/F errors result in catalyst breakthrough and higher emission levels, making it difficult to meet the stringent emission regulations.
In an attempt to improve the performance of the FAOS control system, it has been proposed to use a model-based feedback controller. In the model-based system, the inputs to the system to be controlled are also applied to a model. The model is adaptively updated based upon a comparison between the output of the model and the output of the actual system. The output of the model is used to generate a real-time corrective signal to rapidly compensate for potential, or present, system response errors. The accuracy of the model is important to the success of the model-based system.
The three-way catalyst (TWC) model is one of the principal elements affecting the accuracy of the model-based system. In this regard, there are two types of TWC models. The first is known as a first principles model and is based on reaction kinetics and gas adsorption/desorption on the catalyst surface. The first principles model is usually expressed in coupled partial differential equations that are computationally too complex to be used for control purposes. An example of this type of TWC model is described in the 1994 SAE paper 940934, xe2x80x9cTransient Modeling of 3-Way Catalytic Convertersxe2x80x9d by K. N. Pattas, et al.
The second type of TWC model is a control-oriented model. Models of this type, that have been proposed thus far, are generally single state models that are unable to capture the complex dynamic behavior of the TWC over a wide range of operation. Examples of this type of TWC model are described in the 1996 SAE paper 961038, xe2x80x9cIndirect Adaptive Control of a Three-Way Catalystxe2x80x9d by E. Shafai, et al. and the 1996 SAE paper 961188, xe2x80x9cModel-Based Fuel Injection Control System for SI Enginesxe2x80x9d by M. Nasu et al. FIG. 2 is an example of the output of the control-oriented model 110 described by Shafai et al. in comparison to measured A/F 120 at the TWC output. The triangular pattern of the model""s output only crudely approximates the measured A/F.
The present invention is a three-way catalyst model that accurately represents the dynamic response of the three-way catalyst over its entire range of operation. The model of the present invention divides the TWC response into distinct sub-regions that are each represented by its own unique sub-model. With the present invention, it is possible to build a highly accurate TWC model covering a wide range of operation without overly complex computations.
Each of the sub-regions of the TWC model in accordance with the present invention has a sub-model that may have different levels of complexity and may be based on different modeling techniques. The TWC model of the present invention uses a hybrid state space, containing continuous and discrete states, to define the state of the TWC and to determine when to apply the suitable sub-model for each sub-region.
One object of the present invention is to accurately model the complex behavior of a dynamic system. Another object of the present invention is to model the complex behavior of a Three-Way Catalyst (TWC). Yet another object is to accurately model the TWC without unduly complicated partial differential equations.
A further object of the present invention is to provide a TWC model that has a hybrid state space including continuous and discrete states. It is yet a further object of the present invention to divide the TWC behavior into distinct sub-regions, assign unique attributes to each sub-region, and model each sub-region with an appropriate sub-model.
Yet a further object of the present invention is to define metrics that help quantify and understand the behavior of an automotive TWC. It is still a further object of the present invention to use the novel TWC model in a control feedback loop for an engine A/F control system. It is yet another object of the present invention to isolate and define different dynamic phenomena in the TWC dynamic behavior.
Other objects and advantages of the invention will become apparent upon reading the following detailed description and appended claims, and upon reference to the accompanying drawings.